As wavelength division multiplexed (WDM) networks containing large numbers of wavelength channels are becoming more common, the need for multi-wavelength/multi-frequency sources is increasingly important. Multi-frequency sources, such as tunable lasers, have the ability to tune to different frequencies either continuously over some allowable range or at discrete wavelength values. Since each channel in a WDM optical communication system operates at a distinct wavelength, multi-wavelength sources, such as tunable lasers, are essential to relieve inventory and stockpiling issues associated with systems with a discrete source for each wavelength.
Multi-frequency lasers can be realized by monolithically integrating a frequency routing device, such as a waveguide grating router (AWG), and an array of semiconductor amplifiers (SOAs) into a single laser cavity. Initially, multi-frequency lasers required a separate SOA for each wavelength channel, thereby limiting the maximum channel count. Subsequently though, in improved designs, the total number of required SOAs was reduced to a number proportional to twice the square root of the total channel count.
A disadvantage of the improved multi-frequency laser designs having reduced numbers of SOAs though, is the small fraction of the laser light in the laser cavity that can be extracted, which limits the maximum obtainable output power of these multi-frequency lasers. This problem originates from the fact that in such multi-frequency laser designs it is difficult to bring all channels together in one common output waveguide. Therefore, the power had to be extracted from one of the outer waveguides of the AWG itself, which is able to capture only a small fraction of the lasing power.